Vitamin D

The Fat Soluble Vitamins · Vitamin A · Vitamin D · Vitamin E · Vitamin K The Water Soluble Vitamins · The B-Group Vitamins · Thiamine - Vitamin B1 · Riboflavin-Vitamin B2 · Pantothenic Acid - Vitamin B3 · Biotin - Vitamin B4 · Nicotinic Acid (Niacin) - Vitamin B5 · Pyridoxine - Vitamin B6 · Cyanocobalamin - Vitamin B12 · Ascorbic Acid - Vitamin C

Vitamin D and it's derivatives are all sterols.  Certain sterols when exposed to ultraviolet light undergo small structural changes that are of importance in nutrition, they are called vitamin D2 and D3.  Calciferol (Vitamin D2) is manufactured by existing ergosterol, a sterol found in fungi and yeast’s, to the action of ultraviolet light.

Vitamin D3 is the natural form of Vitamin D.  It is produced by the ultraviolet irradiation of 7-dehydrocholesterol, a sterol widely distributed in animal fats.  Activation of 7-dehydrocholesterol takes place in humans when the surface of the body is exposed to sunlight.  The liver oils of fish are a very rich source of Vitamin D3.

Physiological Action
The amino action of Vitamin D in the body is to promote the absorption of calcium and phosphates from the GI tract.  Vitamin D by facilitating the absorption of calcium ensures a sufficient supply in the extracellular fluids rather than growing points of bones.  Here calcium comes in contact with the phosphates, thus calcium phosphate is formed and used by the osteoblasts to make new bone.

Deficiency
Vitamin D deficiency alters the processes involved in the growth of bones.  Rickets is the childhood disease due to lack of Vitamin D.  In adults the comparable deficiency disease is osteomalacia, in which calcium is withdrawn from the bones.

The bones lose their minerals and protein understructure and become porous, weak and easy to break.  Deafness can result from Vitamin D deficiency, because sounds are transmitted to the brain along the tiny bones of the ear and these degenerate when Vitamin D is lacking.
Toxicity

Vitamin D is the most toxic of all vitamins, as little as 4-5 times the RDI can cause detrimental effects such as diarrhoea, headaches and nausea.  If overdoses continue the vitamin will increase the mineral levels of the blood to dangerous extremes, which can be fatal.

Vitamin D does not necessarily have to be taken in the diet, it can be manufactured in our skins as previously shown by the action of sunlight.

However, if exposure to sunlight is limited by excessive clothing, limited sunlight itself or in large cities where big buildings prevent sufficient exposure or as in dark skinned people who require greater exposure because of their skins natural filtering effect, rickets can still be a problem.

Dietary Sources
The most significant food sources are oils, eg. cod-liver oil, butter, cream, egg yolks and liver.  In many countries all oil products are fortified with Vitamin D, as are breakfast cereals and margarine.  Strict vegetarians, if they do not receive sufficient sun exposure, require supplements of Vitamin D.

Recommended Dietary Intakes

• 10 micrograms (400 IU) for children
• 5 micrograms (200 IU) for adults and a further 5 micrograms during pregnancy and lactation.